Abstract:We studied two mating systems for a large-scale breeding colony of cynomolgus monkeys: (i) a 3-day timed system that keeps one female and one male together in a cage for 3 days beginning 11 days after menstruation, and (ii) a 7-day timed system beginning 9 days after menstruation. Regardless of the mating system used, we propose a practical definition of an infertile female as one that does not become pregnant in up to six pairings. If we eliminate these animals from the breeding colony, the pregnancy rates would be 3.6 times higher. Eliminating infertile females from a breeding colony is important in order to save labor, time and cost and to maintain a healthy breeding colony.

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Retrospective analyses were made on the relationship between fetal position and stillbirth, using 703 pregnant cynomolgus monkeys. Incidence of the breech position was 59.1 % to 12 weeks of gestation. The rate decreased stepwide to 10.4% on the day before delivery. Twenty‐one (65.6%) of 32 monkeys who were in the breech position on the day before delivery had stillbirths, whereas only one stillbirth occurred among 275 monkeys whose fetuses were in the cephalic position.